1. Field of the Invention
The present invention relates to a fuel-injection control system for an internal combustion engine adapted to revise engine acceleration on the basis of the amount of air intake.
2. Description of the Prior Art
FIG. 6 shows a general arrangement of a conventional fuel-injection control system employing an air flow-rate sensor (referred to as an AFS hereinafter) adapted to detect the amount of intake air sucked into an internal combustion engine. The fuel-injection control system illustrated includes an air cleaner 1, a hot-wire type AFS 2, a throttle valve 3 for controlling the amount of intake air sucked into an engine, a throttle sensor 4 operably connected with the throttle valve 3 for picking out the opening degree of the throttle valve 3 as a voltage signal, a surge tank 5, an intake manifold 6, an intake valve adapted to be operated by an engine crank shaft (not shown) through a valve operating mechanism (not shown), a plurality of engine cylinders 8 only one of which is actually illustrated for simplification, a fuel injector 9 provided for each engine cylinder 8, and an electronic control unit 10 (referred to as an ECU hereinafter) adapted to control the amount of fuel injected by each of the fuel injectors 9 in relation to the amount of intake air sucked in by the corresponding one of the engine cylinders 8 in such a manner as to provide a predetermined air/fuel ratio. The electronic control unit 10 functions to determine the amount of fuel injected by the respective fuel injectors 9 on the basis of control signals from the AFS 2, a crank-angle sensor for detecting the rotation angle of the engine crank shaft (not shown), a starter switch 12, a temperature sensor 13 for detecting the temperature of engine coolant, and the throttle sensor 4, and the electronic control unit 10 also controls the pulse width of an electric pulse signal for each of the fuel injectors 9 in synchronization with a signal from the crank-angle sensor 11.
FIG. 7 shows various wave forms of control signals for explaining a fuel injection process during engine acceleration in accordance with the conventional hardware arrangement as illustrated in FIG. 6. In FIG. 7, the engine is raced or accelerated rapidly from no load 750 rpm with the throttle valve 3 being operated from a fully closed to a fully opened state. FIG. 7(a) shows the output signal of the AFS 2, and FIG. 7(b) shows the output signal of the crank-angle sensor 11 in which the falling points are TDC (top dead center) and the rising points are BDC (bottom dead center) with an interval between the adjacent TDCs being equal to a crank angle of 180.degree.. FIG. 7(c) shows the output signal of the throttle sensor 4 which is sampled at intervals of a .DELTA.t time period so as to obtain a differential opening .DELTA..theta.. In this connection, when the differential opening .DELTA..theta. is equal to or larger than a predetermined value, that is when .DELTA..theta. .gtoreq..theta..sub.o, there will be issued special pulses which are separate from injection pulses and synchronized with a signal representative of both the crank angle and the number of revolutions of the crank shaft (not shown), as illustrated by pulses designated by the shaded areas in FIGS. 7(d) through 7(g). In addition, FIGS. 7(d) through 7(g) show pulse forms of injection signals in respective engine cylinders of a four-cylinder internal combustion engine in which fuel for the respective engine cylinders is injected simultaneously from the respective fuel injectors 9.
It is considered that the above-described special pulses are essential for today's finer evaluation of engine response on the points of running performance of a vehicle and pickup during acceleration of an engine. However, provision of a throttle sensor for revising engine acceleration is uneconomical and it is desirable to effect such revision of acceleration by utilizing an output signal from the AFS. In cases where the same processing as that with the throttle sensor is effected in the AFS during acceleration of an engine, the full throttle range (that is the vibration range in FIG. 7(a)) is entirely judged to be acceleration due to a pulsating or blowback phenomenon.
To avoid this, it has been considered to average the signals from the AFC between the adjacent TDCs, and then compare the change rates of the averaged AFC signals at respective TDCs.
In this case, however, experiments have showed that the timing at which the respective special pulses are generated must be such that the first special pulse is generated within a period of time of 20 ms after acceleration. But, in this connection, at a rotational speed of the engine of 750 rpm, the interval between the adjacent TDCs is 40 ms, and hence if the acceleration timing is 40 ms, a duration of 20 ms, required for generating the first special pulse, elapses. Accordingly, there is no choice but to employ a throttle sensor for effecting acceleration correction.
Thus, with the conventional fuel-injection control system for an internal combustion engine, an expensive throttle sensor is required for the minute or finer revisions of engine acceleration, as set forth above.